The present invention relates to devices for measuring sound power and, more particularly, to an acoustic intensity measuring device which measures the vector product of sound pressure and particle velocity in an acoustic wave. There has been increasing interest in the recent years in acoustic intensity measurements. Part of this is due to the increased awareness of noise as a hazard in the work place. Devices capable of measuring acoustic intensity provide a means for determining the sound power contribution of a particular source by integrating the acoustic intensity vector over an imaginary surface surrounding the source. U.S. Pat. No. 1,892,644 issued to Harry F. Olson on Dec. 27, 1932 disclosed one of the first systems and apparatus for measuring the energy flow of a sound wave. The Olson apparatus utilized one microphone for measuring sound pressure and a velocity responsive microphone for measuring the average velocity of a sound wave past a point. However, the early measurement attempts such as Olson required complex, specialized instrumentation and were generally only partially successful. More recently, as disclosed in an article entitled "Measurement of Acoustic Intensity Using the Cross-Spectral Density of Two Microphone Signals" by F. J. Fahy, in the Journal of Acoustical Society of America, Vol. 62, pp. 1057-1059, 1977, a significant improvement was shown in intensity measurement with phase development of an intensity formulation in terms of cross-spectral densities. The Fahy method of measurement permits the use of a two-microphone technique. The two microphones or electroacoustic transducers each converts sound pressure into an electrical signal. The microphones are separated by a predetermined distance and the electrical signals are processed to measure the acoustic intensity vector along the line joining them. Letting the microphone signals be P.sub.A and P.sub.B, the average pressure is EQU P=1/2(P.sub.A +P.sub.B)
and the particle velocity is EQU u=(1/pd)(P.sub.B -P.sub.A) dt
at the midpoint between the microphones, where p is the density of the fluid and d is the microphone separation. By definition, the acoustic intensity vector is EQU I=p.multidot.u
Measurement devices such as the sound intensity analyzing system manufactured by Bruel and Kgaer Type 3360, Sound Intensity Analyzing System, employ the two-microphone cross-spectral acoustic intensity measurement method. Devices such as the Bruel and Kgaer Type 3360 Sound Intensity Analyzing System process the electrical signals from the microphones utilizing digital components. Such devices are not readily portable and are quite expensive.
The acoustic intensity measuring device of the present invention is portable, lightweight and inexpensive and it is readily adaptable for use in the field such as a factory or hospital for measuring acoustic intensity wherever desired.